64 REPORT — 1853. 



Zoology. 



On the Structure of Hydra viridis. 

 By Professor Allman, M.D., M.R.I.A. 



The author had been recently led to examine Hydra viridis, with special reference 

 to its alleged non-cellular structure, as maintained by Ecker, and has arrived at con- 

 clusions entirely opposed to those of the German physiologist. 



Hydra viridis, like all the other Hydroid zoophytes, is composed throughout]of two 

 distinct layers ; to the external of these the author gives the name of ectoderm, and 

 to the internal that of endoderm. When examined under slight pressure and with a 

 power of about 100 diameters. Hydra viridis may be seen to possess throughout the 

 whole thickness of its substance a multitude of clear spaces, which at first look like 

 cells, but by a careful examination may be satisfactorily proved to be mere vacuoles. 

 So far Ecker is right in asserting the existence of vacuolae in the tissues of Hydra, 

 but he is (juite wrong in his opinion as to the relation of these vacuolae to the inter- 

 vening substance. The vacuolee of the endoderm may be seen to be separated from 

 one another by multitudes of green spherules, to which the characteristic colour of the 

 species is due ; and it is the appearance thus presented which has led to the erro- 

 neous belief that the spherules are imbedded in a continuous semifluid matter in 

 which the vacuolce are excavated. 



By a little manipulation, however, the tissue of the endoderm may without 

 difficulty be broken up into detached portions, each almost always containing one, 

 or occasionally more of the clear vacuolse, surrounded by green granules, and isolated 

 hy a distinct though extremely delicate cell-membrane. It is therefore evident that 

 the substance which separates the vacuolae of the endoderm is not continuous, but 

 is contained as cell-contents in true cells, that the vacuolae are excavated in this 

 protoplasm, and that the green spherules are imbedded in it. The cells themselves 

 appear to possess but a verj' weak union among one another ; they are easily sepa- 

 rated by a slight force, and on becoming free, immediately assume a spherical figure 

 without any trace of their having been previously united into a tissue. 



Those endodermal cells which present a free surface in the gastric cavity are 

 deficient in green spherules, but contain a large vacuola, with one or more brown 

 granular masses, which appear to be immediately included in a small secondary cell, 

 in which they are probably elaborated by a true secretory action; they may perhaps 

 he fairly assumed as representing the biliary secretion in the higher animals. Whether 

 the cells, however, which thus constitute the gastric surface of the endoderm are 

 entirely destitute of green spherules, the author could not positively assert : it is 

 certain, that in the disintegration of the endoderm, several cells are liberated con- 

 taining both green spherules and brown granular masses, the latter immediately in- 

 cluded in minute secondary cells, but from what part of the endoderm they were 

 derived he could not determine. The cells which thus constitute the immediate 

 walls of the stomach, cannot be viewed as forming a third layer distinct from the 

 endoderm. 



The green spherules possess an exceedingly definite form, and the author was of 

 opinion that they must be viewed as cells. They present in their interior a lighter 

 coloured space, which appears sometimes circular, sometimes somewhat fiask- 

 shaped, and sometimes triradiate, a difference perhaps depending on the difference of 

 aspect in which it presents itself to the eye. 



The structure of the ectoderm differs in no essential point from that of the endo- 

 derm, except in the fact that its component cells are totally destitute of green sphe- 

 rules and brown granules, while one or more thread-cells, each immediately enclosed 

 in a secondary cell, constitute their characteristic contents ; besides the thread- 

 cells they contain homogeneous colourless contents with vacuolae. 



Hydra is certainly destitute of cilia on any part of its external or internal surface, 

 and yet weak currents may be distinctly seen in the fluid in contact with separate 

 portions of the endoderm. It appeared to the author that the true cause of these 

 currents is to be sought for in certain chemical changes, which, by virtue of their 

 vital endowments, these cells, Uke secreting cells generally, effect in the fluid in con- 

 tact with them. 



In the tissues of Hydra viridis nothing beyond the elements now mentioned could 



